From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Issues and potential improvements for the 20/25 model, focusing on the exhaust, valves, and cooling system.
Identifier | ExFiles\Box 106\5\ scan0157 | |
Date | 25th April 1936 | |
-2- (2) The 20/25 exhaust system is much more restricted than the Bentley exhaust system, which again puts up the exhaust valve temperature. (3) We cannot afford to run austenitic valves at very large clearances on the 20/25 because if we do so we may run into noisy valve gear. Run at ordinary clearances up to .007", the austenitic valve will be using the full toe of the camshaft at flat out speeds. Incidentally the present Bentley exhaust valve has not a very large margin of safety, we have broken a number on the test bed. In case the car running in France bumps into trouble again, there are several possible ways of improving the conditions under which the exhaust valve operates :- (1) The water circulation is poor in the 20/25 cylinder head and always has been so. You brought the matter up again the other day, and we altered both the Peregrine and J-111 experimentally, but for some reason it has never got us into trouble on production on the 20/25. On the new design of cylinder head, the deficiencies in circulation through the head seem to be emphasized, though there seems no obvious reason why they should be materially different from the non-turbulent head. Anyhow, we are running the car in France with most of the front circulation holes to the cylinder head blocked up, which reduces the difference in temperature between the rear and the front cylinder from 8° to 1°. We think it would be as well to make this alteration on production in parallel with our tests in France because we are definitely in trouble and it is difficult to see how it can be other than an advantage. (2) There is now an american silchrome steel which has practically the same high temperature strength as KE-965. We are experimenting with this stellited, with a view to bringing it in on the 20/25. (3) We think the water passages in the cylinder head could with advantage be brought further down the valve stem. Though the 20/25 is now the same as the Bentley it appears to be about 1/4" worse off than the original 20/25 cylinder head. With regard to the Bentley valve springs, we may be forced into these to avoid breaking the valves by bouncing, due to the fact that the engine wants to run up to a higher engine speed. We have, however, supplied Hancock with both 20/25 and Bentley valve springs to run, the disadvantages of the Bentley springs are that they will increase the tendency to timing gear rattles and also the tendency of the valve gear to be noisy. | ||